A power inverter which can provide sinusoidal voltage or current is the key apparatus in the field of electrical machine drive and utility interface, such as in renewable energy generation systems and energy storage power conditioning systems. To achieve a higher power rating, each phase of the inverter may be constructed of paralleled phase legs. If two paralleled legs are connected to an output terminal by a magnetic coupling device, such as an “inter-phase transformer”, or a “multi-winding autotransformer”, or an “inter phase inductor”, the output terminal of each phase will have a multilevel staircase waveform, which is closer to the desired sinusoidal waveform. Therefore, the inverter will require smaller magnetic components while still providing the benefit of higher dynamic response.
A magnetic coupling device between the paralleled legs of the inverter is commonly referred to as an “inter-cell transformer” (ICT). Additionally, an inverter that has multiple phase legs coupled with inter-cell transformers (ICTs) is commonly referred to as a “parallel multilevel inverter”.
At least two categories of parallel multilevel inverters are known in the art. The first category is a parallel five-level (P5L) inverter, as shown in FIG. 1. And the second category is a parallel seven-level (P7L) inverter, as shown in FIG. 2.
Both P5L, and P7L inverters can be constructed with either NPC (Neutral Point Clamped) phase legs as shown in FIG. 3A or T-type phase legs, as shown in FIG. 3B. The conventional modulation method for P5L and P7L inverters is to interleave the carrier waveforms of each phase leg. As shown in FIG. 4, to interleave the carrier waveforms in a P5L inverter, one set of carriers in a phase leg is shifted by 180 degrees from the other set of carriers in the same phase. In a P7L, the shifted angle among three phase legs in the same phase is 120 degrees. The benefit of this carrier-interleaved modulation is that it can achieve multilevel outputs and magnetic flux balancing for ICTs, simultaneously. Under carrier-interleaved modulation, the volt-seconds applied to ICTs is naturally balanced within a switching period. This is an important feature because windings of an ICT are usually strongly coupled, if the volt-seconds applied to an ICT is unbalanced, the magnetic core of the ICT may quickly be saturated, resulting in an over current fault.
However, the problem with carrier-interleaved modulation is that it also limits the possibility of optimizing the output voltage spectrum. A better optimization of the output voltage spectrum can benefit the design of the inverter in various aspects including: output filter size reduction, ground leakage current suppression and EMI noise attenuation.
In a five-level (P5L) carrier based pulse-width-modulation (PWM), there are carrier disposition methods, namely PD (Phase Disposition), POD (Phase Opposition Disposition) and APOD (Alternate Phase Disposition). Each carrier disposition method generates a unique voltage spectrum. In engineering practice, each voltage spectrum provides advantages for specific applications. However, in a P5L PWM created by interleaving two sets of three-level (3L) carriers, which is commonly used in conventional P5L inverters, no matter how the carriers are disposed, the final output waveforms all have the same voltage spectrum.
Another related aspect is the closed-loop magnetic flux balancing issue for parallel multilevel inverters. Although volt-seconds applied on the ICT are balanced in the modulation, closed-loop balancing is still needed to deal with transient events and parameter unbalances. In the prior art, based on carrier interleaved modulation, closed-loop balancing is achieved by sampling the differential current within one phase and feeding it back into a circulating current control loop that controls the difference in modulation waveforms between phase legs within one phase. As this method doesn't control the exact switching time, the control bandwidth should be much lower than the switching frequency to avoid interfering with the output waveform. Therefore, this method is able to control static or slow rate unbalance. As such, the prior art method doesn't have effective control during fast dynamics. As a result, additional design margin has to be reserved for the ICTs.
Accordingly, what is needed in the art is a modulation method for parallel multilevel inverters that allows optimization of the output voltage spectrum while still maintaining balanced volt-seconds applied on the inter-cell transformers. It is also important that the modulation method provide access to fast magnetic flux rebalancing control.